Update How to Run Tests instructions

Replace PhantomJS with ChromeDriver

Author: ascholerChemeketa

README.rst

@@ -162,7 +162,7 @@ Our goal is to have unit tests which rely on Selenium (a library that helps simu
 
 **To run tests:**
 
-* Make sure the directory containing the PhantomJS executable is in your ``PATH`` environment variable. e.g. ``PATH=$PATH:path/to/virtualenv/directory/where/it/is/here`` at your command line (or edit your ``.bash_profile``). It should live in the site-packages/selenium/webdriver/ directory of your virtualenv.
+* Make sure the directory containing the ChromeDriver executable is in your ``PATH`` environment variable. e.g. ``PATH=$PATH:path/to/chromedriver`` at your command line (or edit your ``.bash_profile``).
 
 * Check out the existing tests, e.g. the ``test_question.py`` file that tests the Question directive, which you can find at the path ``/runestone/question/test/test_question.py``, for an example.